Heretofore, there is known such a secondary battery including a flat electrode wound body as disclosed in Patent Document 1 for example. This electrode wound body includes electrode plates in each of which a current collector made of a metal foil is coated with an electrode active material, excepting a part of the foil in its width direction. Positive and negative electrode plates and separators are wound together in overlapping relation, forming the electrode wound body. Further, in this electrode wound body, a positive current collector is connected to a positive terminal and a negative current collector is connected to a negative terminal is connected, respectively. Patent Document 1 discloses an electrode wound body in which flat-plate-like electrode terminals are connected respectively to current collectors by welding.
When a secondary battery is charged and discharged, a certain degree of heat is generated in the secondary battery. If that heat stays inside the secondary battery, it is undesirable because it may decrease electric performance of the battery. In the secondary battery, therefore, the heat generated in the electrode wound body is allowed to transfer from the current collectors to the outside of the battery via the electrode terminals. There is also another secondary battery in which a heat radiating member or the like is placed on the outside of a battery case.
For instance, in a lithium ion secondary battery, preferably, a current collector and an electrode terminal for positive electrode are both made of aluminum and a current collector and an electrode terminal for negative electrode are both made of copper. In the positive electrode, accordingly, the aluminum current collector and the aluminum electrode terminal are connected. For this connection, ultrasonic welding is used in many cases. In the negative electrode, on the other hand, the copper current collector and the copper electrode terminal are connected. For this connection, therefore, resistance welding is used in many cases. Accordingly, the heat generated in the battery will be transferred to the electrode terminals via their weld portions.